PhpED recognizes file types by their extensions. By default, it understands the following extensions as php scritps:
*.php;*.inc;*.php3;*.php4;*.php5;*.phtml;*.shtml

You can change this list in Tools->Settings->Associations

Should be also known that while in HTTP/SRV run mode PHPED works with SRV web server and it supports its own list of associations. This list of extensions (one by one) and respective cgi handlers (php.exe) can be updated in Tools->Settings->Srv web server. This list is also used in Local CGI mode. As for HTTP/3rd party web server mode, it's up to Apache, IIS or whatsoever webserver you run your scripts under... Read their documentation on how to associate them with php scripts.